Access your X-il.jp (IIJmio セーフティメール) Account from an Email Program using IMAP

X-il.jp supports IMAP / SMTP

This means you don't have to use X-il.jp's webmail interface! You can
check your email and send messages using other email
programs (like Mailspring, Outlook Express, Apple Mail, or Mozilla Thunderbird).
Using desktop email applications can improve your workflow. They typically
offer more options, and your email is still available on your
computer when you're disconnected from the internet.